1. Technical Field
The present invention relates to a magnetic head having an exchange-coupled free layer with out-of-plane magnetization.
2. Related Art
In the related art magnetic recording technology such as hard disk drives, a head is equipped with a reader and a writer that operate independently of one another. The reader includes a free layer, a pinned layer, and a spacer between the pinned layer and the free layer.
In the reader, the direction of magnetization in the pinned layer is fixed. However, the direction of magnetization in the free layer can be changed, for example (but not by way of limitation) depending on the effect of an external magnetic field, such as the recording medium.
When the external magnetic field (flux) is applied to a reader, the magnetization direction of the free layer is altered, or rotated, by an angle. When the flux is positive, the magnetization of the free layer is rotated upward; and when the flux is negative, the magnetization of the free layer is rotated downward. If the applied external magnetic field changes the free layer magnetization direction to be aligned in the same way as pinned layer, then the resistance between the layers is low, and electrons can more easily migrate between those layers
However, when the free layer has a magnetization direction opposite to that of the pinned layer, the resistance between the layers is high. This high resistance occurs because it is more difficult for electrons to migrate between the layers.
FIG. 1(a) illustrates a related art tunneling magnetoresistive (TMR) spin valve for the CPP scheme. In the TMR spin valve, the spacer 23 is an insulator, or tunnel barrier layer. Thus, the electrons can cross the insulating spacer 23 from free layer 21 to pinned layer 25 or verse versa. TMR spin valves have an increased magnetoresistance (MR) on the order of about 50%.
FIG. 1(b) illustrates a related art current perpendicular to plane, giant magnetoresistive (CPP-GMR) spin valve. In this case, the spacer 23 acts as a conductor. In the related art CPP-GMR spin valve, there is a need for a large resistance change ΔR, and a moderate element resistance for a high frequency response. A low coercivity is also required so that a small media field can be detected. The pinning field should also have a high strength.
FIG. 1(c) illustrates the related art ballistic magnetoresistance (BMR) spin valve. In the spacer 23, which operates as an insulator, a ferromagnetic region 47 connects the pinned layer 25 to the free layer 21. The area of contact is on the order of a few nanometers. As a result, there is a substantially high MR, due to electrons scattering at the domain wall created within this nanocontact. Other factors include the spin polarization of the ferromagnets, and the structure of the domain that is in nano-contact with the BMR spin valve.
In the foregoing related art spin valves, the spacer 23 of the spin valve is an insulator for TMR, a conductor for GMR, and an insulator having a magnetic nano-sized connector for BMR. While related art TMR spacers are generally made of more insulating metals such as alumina, related art GMR spacers are generally made of more conductive metals, such as copper.
In the related art, it is necessary to avoid high interlayer coupling between the pinned layer and the free layer, so that magnetization of the free layer is only affected by the media magnetic field itself during the read operation. High interlayer coupling has the undesired effect of negatively affecting the output read signal. For example, the signal asymmetry and the hysteresis are substantially increased. This effect is disclosed in Cespedes et al. (Journal of Magnetism and Magnetic Materials, 272-76: 1571-72 Part 2, 2004).
In the current confined path-CPP head, the spacer is made of non-magnetic, conductive areas that are separated from one another by an insulator, such as Cu—Al2O3. Accordingly, interaction between the free layer and the pinned layer is increased as the thickness of the layers decreases, especially in the cases of GMR and TMR. For example, in the TMR head, the insulating spacer is made very thin to reduce overall device resistance. This reduced TMR spacer thickness also causes the creation of pinholes between the free layer and the pinned layer, which results in an increased interlayer coupling.
In the case of BMR, the interlayer coupling increases as a function of the nanocontacts (which are direct connections) present in the spacer between the free layer and the pinned layer. When the free layer and the pinned layer have opposite directions, a magnetic domain wall can be created. As a result, a high MR ratio can be obtained, with strong electron scattering. For example, when the free layer and the pinned layer are connected by Ni magnetic nanoparticles embedded in an alumina matrix of the spacer, a high interlayer coupling that is greater than about 100 Oersteds (possibly about 200 Oersteds) occurs. As a result, the transfer curve (i.e., voltage as a function of the external magnetic field) becomes asymmetric, and the output signal is substantially reduced.
FIG. 2 illustrates a related art free layer (e.g., 21) from its top view. When a high sense current flows in the direction 51 perpendicular to the plane of the free layer (i.e., into the page of FIG. 2), a magnetic field is generated according to Ampere's law. This magnetic field will create a curling distribution of free layer magnetic moments. (as represented by the arrows 53). Accordingly, there is a need to solve this related art curling problem.
In the related art, this curling problem can be solved by introducing a side stabilizer. However, if the magnetic field generated by the side stabilizer is too high, then the sensitivity of the free layer is reduced. Consequently, free layer rotation will be substantially prevented. Further, the free layer magnetization cannot rotate easily under the media magnetic field. Thus, there is an unmet need to generate a strong magnetic field due to the related art side stabilizer, for example a hard bias stabilizer.
As shown in FIG. 3, in the related art, a single free layer with perpendicular magnetic anisotropy has been proposed. The free layer 21 and the pinned layer 25 are separated by the spacer 23. In this related art scheme, the free layer is made of Fe/Pt, Fe/Pd, or alloys thereof. However, this related art proposal has various problems and disadvantages. For example, but not by way of limitation, this related art proposal has free layer materials that suffer from small spin polarization due to the content of Pt and/or Pd, which is a high anisotropy material.
Thus, there is an unmet need in the related art for a low anisotropy and high spin polarization material that overcome the aforementioned related art problems.